Confusing but moody and unsettling, this rather nutty thriller begins to fall apart the more it tries to explain its premise, but it maintains a terrific sense of suspense right to the end.

Gina (Headey) is a radiologist in a London hospital whose life is thrown out of balance after a mirror breaks during a family dinner. The next day she sees a double of herself and follows her home, discovering a photo of the stranger with her father (Jenkins). Her doctor (Armstrong) thinks she's bruised her brain in an accident. But then her boyfriend (Poupaud) starts acting just a little bit strangely. And things start turning bizarre for Gina's brother (Newman) and his girlfriend (Duncan) as well. Is the world being invaded by mirror-dwelling body-snatchers?

Writer-director Ellis creates a wonderfully disorienting atmosphere, using references to classic horror movies to keep us off balance while dropping subtle hints about what's going on here. The camerawork is creepy and insinuating, with music that constantly suggests that something terrifying is happening, even though we're often unsure what that might be. In other words, the film develops a terrific sense of tension while only rarely putting something scary on screen.

It definitely helps to have such a solid cast. Headey holds the film together with an internalised performance that really catches Gina's growing suspicions. And even though the film essentially wastes the gifted Jenkins, Poupaud and Thomsen (as her therapist), they manage to inject solid subtext in their scenes. These must have been fun roles to play, especially as the characters shift into slightly not-right states, pushing Gina even further down the rabbit hole, as it were.

In the end, there's not enough to the film to make it a classic, but it's original enough to keep us gripped. Ellis' deliberately vague approach is superb in this sense, as he's able to make minor, everyday things seem deeply ominous simply because tiny details just aren't quite right. He also cleverly plays with memory and identity through the use of mirrors, water and blood. And if the story doesn't really reach a satisfying conclusion, it will definitely get under your skin.